The present invention relates generally to power line communication systems and, more particularly, to a circuit that permits communication signals to bypass an open switch in a power distribution circuit while preventing the passage of potentially lethal 60 Hz line current around the open switch.
Electric utility companies utilize power line communication systems to communicate between a central station and remote devices, such as electric meters and load management terminals that are equipped with receivers or transceivers. In a system of this type, a central station is generally configured to transmit signals on the power line distribution system to communicate with a plurality of remote stations. The remote stations can be electric meters or load management terminals that are located at consumers' residences. The load management terminals are typically configured to shed interruptible electric loads, on command from the central station, during periods of peak demand. The signals emanating from the central station can be load shed commands or requests for electrical energy consumption data. In the latter case, the central station transmits a command that instructs a particular remote terminal to provide its consumption data by return transmission. The remote stations can be either one-way or two-way devices wherein the one-way devices are capable of following command instructions, such as load shed commands, and the two-way devices are capable of transmitting messages to the central station in response to received commands.
Although many different types of communication techniques are possible for use with a power line communication system, one known technique emloys a high frequency signal which is modulated by a baseband data signal containing a coded message. A high frequency signal can be, for example, 12.5 kHz and may be modulated by the baseband data signal in a coherent phase shift keyed (CPSK) system. The modulated signals are demodulated by the receiving stations. U.S. Pat. No. 4,311,964, which issued to Boykin on Jan. 19, 1982, describes a coherent phase shift keyed demodulator that is usable in a power line communication system. U.S. Pat. No. 4,379,284, which issued to Boykin on Apr. 5, 1983, discloses another coherent phase shift keyed demodulator. Load management terminals (LMT's) that can be used in such a power line communication system are disclosed in U.S. Pat. No. 4,402,059, which issued to Kennon et al. on Aug. 30, 1983, and U.S. Pat. No. 4,130,874, which issued to Pai on Dec. 19, 1978.
U.S. Pat. No. 4,250,489, which issued to Dudash et al. on Feb. 10, 1981, discloses a distribution network communication system having branched connected repeaters and U.S. Pat. No. 4,427,968, which issued to York on Jan. 24, 1984, discloses a distribution network communication system with flexible message routes. When messages must travel a significant distance on the power line, it is a known technique to utilize a repeater that is capable of receiving a transmitted message and retransmitting that message in order to provide a stronger signal for receipt by remote receivers. When the remote receivers are located at significant distances from the central station, one or more repeaters is used to assure that transmitted signals are not attenuated beyond the capability of the receivers. U.S. Pat. No. 4,032,911, which issued to Melvin on June 28, 1977, discloses a signal repeater for power distribution line communication systems.
Electric utility companies employ switches in the power line distribution network that permit portions of the network to be disconnected when a fault is detected within that portion. For example, if part of a power line is damaged due to an electrical storm, ice damage or damage to an electric utility pole, appropriate switches are open to disconnect that portion of the power line distribution system from the main network to facilitate repair. When this type of power line distribution switch is opened, the power line communication system which utilizes the power line would normally experience a discontinuity that prevents signals from being transmitted to or received from remote devices in that portion of the power line distribution network. When the power line distribution switch is opened, power is not necessarily removed from one side of the switch. Although, under certain circumstances, an open power line distribution switch will result in the deprivation of power from one side of the switch, this situation may not arise if that side of the switch is provided power from another source in the power distribution network. Even though that portion of the power line distribution network can be deprived of electrical power during periods when the switch is open, the remote devices could otherwise receive and transmit messages if they are provided with a battery backup system. However, the discontinuity in the power line that is caused by the open switch would normally prevent communications between the remote terminals and the central station by depriving the remote devices of a signal transmission path to the central station.
The present invention provides a means for communicating signals around open switches of a power line distribution network. It permits high frequency signals, such as the 12.5 kHz coherent phase shift keyed signals described above, to bypass the open switch while preventing lower frequency current, such as the 60 Hz power distribution frequency, from passing around the open switch. The prevention of the 60 Hz line current is critical since it would otherwise be dangerous if this power distribution current is allowed to bypass the open switch. The bypass of the 60 Hz current around the open switch would defeat its essential purpose of disconnecting the affected portion of the network from the main power source during repair procedures.
The present invention comprises two basic portions. Each portion is provided with a transformer that has a primary and a secondary winding. An inductor is connected electrically in parallel with the primary winding of the transformer and a first terminus, or termination, of the primary winding of the transformer is connected to ground. A capacitor is connected electrically in series between a second terminus of the primary winding of the transformer and the inductor. One side of the power line circuit is connected electrically to the capacitor, between the capacitor and the inductor. Another similarly configured apparatus comprises a transformer, an inductor and a capacitor connected in the manner described above. The capacitor of this second portion of the present invention is connected to the opposite side of the switch and the secondary windings of these two transformers are connected together. This configuration permits high frequency communication signals to pass from one side of the open switch to the other while blocking lower frequency currents, such as the 60 Hz line current. The present invention also permits a transceiver to be connected electrically to the secondary windings of the two transformers to receive power line communication signals and retransmit those signals following amplification. This type of transceiver would operate as a repeater.
The present invention blocks 60 Hz line current from passing around an open switch in a power distribution network while permitting high frequency power line communication signals, such as a 12.5 kHz signal, to bypass the switch. It permits remote receivers, that would otherwise be disconnected by an open power line distribution switch, to receive signals from a central station and transmit signals to that central station even though an open power distribution switch exists in the power line between the central station and the remote terminals.